One of the critical bits of wisdom central to a skeptical outlook is the realization that our brains are not objective perceivers of reality. Not even close. What we perceive as reality is constructed in an active process that is rife with assumptions and flaws. Everything you take for granted about what you experience as yourself and the outside world is actively constructed by specific brain processes.

There are some assumptions that are so fundamental to your construct of reality that you take them for granted – you are not even aware they are happening. We only know about them from cases where these mechanisms break down. For example, there is specific brain processing that makes you feel as if you are separate from the rest of the universe. If this processing breaks down, you will have the sense of feeling merged or one with the universe. Most people who experience this, usually as a consequence of psychoactive chemicals, feel that they have had a profound experience. In a way they have – they have had a “for the world is hollow and I have touched the sky” moment. They have peeked behind the curtain of their brain’s function and have experienced what it is like to have their brains construct reality in a different manner from what they are used to.

In essence they have had a profound internal experience. Many people however will interpret this as an external experience – that in some meaningful way they have touched the universe or experienced God or something similar. It is no surprise that many cultures have traditions that involve the use of psychoactive drugs in order to induce profound spiritual experiences.

We see this internal/external confusion in many other contexts as well. People who have a hypnagogic hallucination may feel they were abducted by aliens or visited by demons, rather than had a neurological episode of waking dreams. People frequently experience optical illusions (a break down of the visual construction of external reality) and react to what they constructed in their brains as if it were an accurate representation of external reality.

Another example of this overall phenomenon is the fact that our brains perform specific processing in order to create the sensation that we are inside our own bodies, that we own our body parts, and that we have agency or control over these body parts. These are all constructed experiences that can break down with damage to one or more brain structures. Further, as will optical illusions, these brain processes can be tricked into creating an alternate construction.

Henrik Ehrsson of the Swedish medical university Karolinska Institutet has published a series of studies in which he explores how the brain constructs the sense that we own our own bodies, and experimental conditions in which this process can be tricked to make us feel as if we are not inside our bodies, we do not own parts of our body, that we occupy another body (real or virtual), or that we own non-existent body parts. Not only is this absolutely cool, but what better demonstration of the fact that our realities are constructed by brain processes – and that you cannot absolutely trust what you experience.

In one experiment he was able to induce subjects to feel as if they had swapped bodies with another person. Subjects actually feel as if they occupy another body. This and other studies reveal some information about the process. The brain uses multiple sensory clues – primarily vision and tactile sense – compares these sensations and then uses the comparison to construct the sense of location and ownership. For example, you can see something touching your hand, you also feel that your hand is in the space that is being touched (proprioception) and you feel the touch (tactile sensation). Your brain compares these sensory streams and if they all match up that creates the ongoing sensation that what was touched is your hand.

This creates the opportunity to trick the brain. What Ehrsson has done is use virtual reality goggles or other setups to show a subject either another person’s arm or a virtual arm. That other arm is then touched and the subject’s real arm is touched in the same place at the same time. The brain then assumes that the arm it is seeing must be the subject’s arm, and creates the sensation that it is, in fact, the subject’s arm.

There is a distinct phenomenon of agency – the sense that we control our body parts. Alien hand syndrome is a category of syndromes in which one feels that they do not control their own hand. This too is caused by a disconnect among various sensory streams that do not match. In this case the brain compares the activity in those parts of the brain that plan and execute movement with the sensory information about what the limb actually does. If they match – then that creates the sensation of agency. If that process breaks down (again, usually from a stroke or other brain damage) then the limb feels like it is under alien control.

The theme among these various phenomena – body swapping, phantom limbs, and alien hands – is that the brain is comparing different sensory inputs to see how they match and then uses the comparison to create the distinct but related sensations of occupying a physical space and body, owning body parts, and controlling body parts.

The implications of this are extremely interesting. For the skeptic it is another example of the fact that our perceptions of reality are constructed. This means that seeing (and experiencing) is not believing. What we experience is largely a fiction. It works well enough most of the time – but not all the time. It can be tricked into incorrect perceptions that we cannot distinguish from accurate perceptions – it all seems really real to our brains.

For neuroscientists a key insight is the fact that our brains have specific processing that involves the comparison of different sensory streams. They are not all separate. Further, there is a separate line of research (which I can discuss in a separate post) that demonstrates that one sensation can affect the perceptions of another. What we see affects what we taste and hear, for example.

And for the futurephile, this research means that one day we can be made to feel as if we occupy that virtual reality character we are playing in the latest video game. Or (more practically) be made to feel as if we own a prosthetic (artificial limb), or that we occupy the robot we are remotely controlling.

Ehrsson is exploring the limits of our ability to body swap. For example, we cannot be made to feel as if we occupy a block of wood. But can we be made to feel as if we occupy a barbie doll? What are the limits of size, shape, number, etc?